A solar cell is a semipermanent and pollution-free energy source which employs sunlight, whereas fossil fuel increases carbon dioxide in the air and greatly deteriorates the global environment. Accordingly, development of various solar cells as an important energy source in further is attempted. A solar cell is commonly used as a solar cell module having a solar cell element sealed by EVA (ethylene/vinyl acetate copolymer) and its front surface and rear surface sandwiched between a transparent glass substrate and a backsheet (rear side laminate).
A backsheet is provided to protect the EVA and the solar cell element, and a film to be used as the outermost layer (a film to be used on the side in contact with the air of a solar cell module, hereinafter referred to as “an outermost film”) is required to have sufficient electrical insulation property (volume resistivity: at least 1×1013 Ω·cm) and moisture-proof property (water vapor permeation at 40° C. under a relative humidity of 90% of at most 1 g/m2·24 h).
Further, since a solar cell module is exposed to the outside for a long period of time, the outermost film of the backsheet is required to have sufficient weather resistance and heat resistance. Specifically, as the exposure by a carbon arc sunshine weather meter (SWM) for from 250 to 500 hours corresponds to the outdoor exposure for one year, the film is required to have such a property that the decrease in the strength by exposure by SWM for 5,000 hours (corresponding to 10 to 20 years) is suppressed to be at most half the initial strength (weather resistance). Further, since the temperature of a solar cell in practical use is at most at a level of 90° C., the film is required to have heat resistance to such an extent that the decrease in strength by use at 90° C. for 20 years is a practically acceptable level.
As the outermost film which has such properties, a fluororesin film using a fluororesin such as ETFE (ethylene/tetrafluoroethylene copolymer), PVF (polyvinyl fluoride) or PVdF (polyvinylidene fluoride) has been known. Among them, an ETFE film and a PVdF film are completely free from a decrease in the strength by hydrolysis even under conditions of 85° C. under a relative humidity of 85% for 1,000 hours. Further, an ETFE film is excellent in the moisture resistance and heat resistance, since the temperature at which the elongation is decreased to half by a heat resistance test for 100,000 hours (about ten years) is from about 150 to about 160° C.
Since an ETFE film is relatively flexible, when used for an outermost film, embossing is applied to the surface so that the gloss of the surface is suppressed and abrasions are not noticeable.
In recent years, the solar cell module is less likely to be installed in roof-integrated system, and is particularly often installed on the slant at an optimum angle so that the transparent glass substrate faces the sun depending on the latitude at the installation site. In such an installation method, a large quantity of reflected light of sunlight is applied to the backsheet at the rear side of the solar cell module, and thus the outermost film is required to have more excellent heat resistance and weather resistance.
Further, for the backsheet, employed is a method of laminating a fluororesin film and an aluminum foil or a moisture-proof plastic sheet to prevent water vapor from entering the solar cell module, since the water vapor permeation cannot sufficiently be suppressed only with a fluororesin film (outermost film). In such a case, with a view to protecting the plastic sheet and an adhesive to be used for lamination from sunlight, the fluororesin film is required to have a transmittance of ultraviolet rays at a wavelength of at most 360 nm of less than 1%. Further, reduction of the transmittance of ultraviolet light at a wavelength of at most 360 nm can be realized by dispersing a white pigment such as titanium oxide in the fluororesin film.
With respect to the improvement in the weather resistance of an ETFE film to be used for agricultural greenhouses and membrane structures, there is a technique to improve the weather resistance of the film by dispersing titanium oxide in an ETFE resin. However, since titanium oxide has photoactivity, the fluororesin is decomposed by the photoactivity when irradiated with light. Accordingly, a film has been proposed having titanium oxide covered with a large amount of silicon oxide or titanium oxide covered with cerium oxide to control the photoactivity, dispersed in an ETFE resin (Patent Document 1).
Patent Document 2 proposes a reduction of the difference (Δ300-100) in the Karl Fischer water content of titanium oxide covered with silicon oxide between at 300° C. and 100° C. to reduce the amount of bubbling water, so as to suppress formation of bubble streaks on a resin composition having titanium oxide incorporated, by water bubbling at the time of forming.
It is preferred to use, for an innermost film (a film on the side in contact with a filler such as EVA) in the backsheet, a film having a high solar reflectance in order that sunlight which leaks to the backsheet side through the solar cell element in the filler is reflected, and the reflected light is returned to the solar cell element again via the transparent glass substrate on the surface. Accordingly, it has been proposed to incorporate a white pigment such as titanium oxide in the innermost fluororesin film of the backsheet for the purpose of improving the solar reflectance (for example, Patent Documents 3 and 4). However, this proposal is for the innermost film of the backsheet, and heretofore, it has not been known to incorporate titanium oxide in the outermost film.    Patent Document 1: JP-A-8-259731    Patent Document 2: JP-A-2006-37090    Patent Document 3: JP-A-2008-085293    Patent Document 4: JP-A-2008-053510